Adjustable flyweight for use in a variable speed belt drive

ABSTRACT

A variable speed belt drive having adjustable mass and moment of inertia camweights is shown for use primarily in conjunction with snowmobile, golf cart, all terrain vehicle and small automobile engines. In one version, the camweight includes a series of perforations or score lines surrounding a cross section of the camweight arm. The perforations define a volume that may be snapped or cut off of the arm with a suitable tool. Also, a series of bores are formed through the arm. In order to increase the mass of the arm, a molten metal or similar flowable material may be poured into one or more of the bores and allowed to cure. In another version of the invention, a reduced cross section arm serves as a base onto which shims are added or reoriented in order to achieve the desired mass and moment of inertia characteristics.

This application is a division of application Ser. No. 08/451,199, nowU.S. Pat. No. 5,562,555 filed on May 26, 1995.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the clutch of a variable speed beltdrive, and more particularly to a flyweight for use in a clutch thatpermits adjustment of the flyweight mass and moment of inertia withoutthe need to remove the flyweight from the driving clutch.

2. Description of Related Technology

A variable speed belt drive is a type of transmission commonly used withinternal combustion engines developing fifty to two hundred horsepowerat shaft speeds of approximately 10,000 revolutions per minute. Suchbelt drives are commonly used on snowmobiles, and permit operation fromlow velocities to speeds of over 100 miles per hour. The belt drivetypically includes a driving clutch having a shaft which is coaxial withthe output shaft of the vehicle's engine. The driving clutch is formedto include a stationary and a fixed sheave, which together define apulley around which a belt may travel. The belt also includes travelaround a driven clutch pulley that transfers the engine's power to theoutput shaft driving the vehicle.

An example of a variable speed belt drive of this type is disclosed inU.S. Pat. No. 3,939,720, issued to Aaen et al. The effective radius ofboth the driving pulley and the driven pulley may be varied, and it isthe ratio of the driving pulley radius to the driven pulley radius whichdetermines the ratio of engine speed to the output shaft rate ofrotation. If the driving pulley radius is small as compared to thedriven pulley, the output shaft will turn at a rate that is slower thatthe engine speed, resulting in a low vehicle speed. As the ratio of thedriving to driven pulley radius approaches 1:1, the output shaft speedwill be approximately equal to the engine speed, and the vehicle speedwill be relatively greater. Finally, as the driving pulley radiusbecomes greater than the driven pulley radius, an overdrive conditionoccurs in which the output shaft is turning at a rate which is greaterthan the crankshaft of the engine.

Ideally, an engine will deliver power in a linear manner and thetransmission will deliver all of the available engine power regardlessof the vehicle's speed or load. Unfortunately, that is not the case witheither real world engines or transmissions. Instead, the typical enginedelivers its maximum power over a narrow range or band of highcrankshaft speeds, with power falling off measurably on either side ofthat band. Ideally, the transmission should permit the engine to operatewithin that band regardless of the load on the engine. Typically, themaximum "power band" has a range on the order of 100 rpm.

In a variable speed belt transmission, the effective radius of both thedriven clutch and the driving clutch are variable and can move while theengine is under power. The driven clutch relies on the combination of apretension spring and a helical torque feedback ramp to exert therequired pressure on the movable pulley sheave to maintain the optimumside load on the belt. While the correct design and adjustment of thedriven pulley sheaves determines the efficiency of the transmissionsystem by properly transferring to the output shaft the engine powerthat is made available to it, the driving clutch must control the enginespeed and keep the engine operating in the "power band" throughout theentire operating range of the transmission.

The driving clutch varies its effective radius by having a movablesheave that decreases the distance between the tapering sheaves and thusincreases its effective radius as the engine speed increases. Movementof the movable sheave occurs because of the force exerted by one or moreflyweights that alter their orientation in response to the centripetalforce caused by rotation of the engine. The mass of the flyweight andits moment of inertia are critical to establishing operation of theengine within the "power band". If the flyweight is too heavy, light ornot properly balanced in its dynamic state, the driving clutch will notbe delivering maximum power to the driven clutch, but will instead beoperating above or below the "power band".

Further, once the proper flyweights are chosen, variations over time inthe engine output, transmission efficiency or vehicle configuration willcause the power band to shift, thereby requiring the replacement of theflyweights. Replacement of the flyweights presents several problems.First, the construction of the movable sheave housing requires that thedriving clutch be substantially disassembled in order to replace theflyweights. Second, there is no readily available method of determiningwhat change to the flyweights is required to achieve the desired result.Hence, one of many fixed weights must be inserted and the clutchreassembled. The vehicle must then be test driven in order to determineif the change in flyweights was helpful. Even if that were the case,there is no way of determining if the substitution of flyweightsachieved the optimum results, and so additional disassembly,substitution, reassembly and testing is required. The entire process isso time consuming that it is seldom properly performed, with the resultthat most snowmobiles, for example, are not actually operating withinten percent of their power band. The resulting inefficiency also causesexcess fuel and oil consumption.

Finally, the sheer number of manufacturer supplied flyweights makes itunlikely that a complete supply will be on hand when needed. Forexample, on page 57 of the Clutch Tuning Handbook by Olav Aaen, 1995edition published by Aaen Performance, 316 Sheridan Road, Racine, Wis.53403, shows thirty four separate drive clutch weights that areavailable for a popular commercial unit. These weights are available intolerances of ±1 gram, meaning that a 50 gram weight could weigh lessthan a stock 49 gram weight. Also, the thirty four weights represent avariety of shapes and thus moments of inertia, all of which must betried empirically in order to approach, but not necessarily achieve,optimum performance.

The ideal solution to this problem is to adjust, rather than replace,the existing flyweights while they are still in place on the clutch. Oneattempt at this approach has been made in the Yamaha YPZ clutch, asdiscussed on page 58 of the Clutch Tuning Handbook. Unfortunately, theYamaha approach is limited to the addition of washers on a flyweight offixed shape. The size of the washers is such that only small changes inweight and moment of inertia can be achieved, and the system assumesthat the subtraction of mass from the original flyweight will not bedesired.

SUMMARY OF THE INVENTION

The present invention addresses the problem of altering the mass andmoment of inertia of flyweights used on a variable speed belt drivewhether or not the flyweights are still mounted on the clutch. Theflyweight or camweight of the present invention uses a relatively lowmass parent weight arm to which numerous shims and ballast of varyingshapes and mass may be quickly added or subtracted. The total mass ofthe weight arm may be left unchanged while the moment of inertia isvaried, the mass itself may be varied without affecting moment ofinertia, or a combination of mass and moment of inertia variation may beutilized. The weight arm is formed to include several threaded holesinto which fasteners may be secured. The fasteners may be formed indifferent shapes and may have differing densities and masses. Thus, insome cases the fasteners by themselves may be sufficient to properlytune the clutch. In other cases, the use of shims having various shapesand masses may be secured to the weight arm with the fasteners. Theshims may also be moved to occupy different positions on the weight arm.Tuning of the clutch can be accomplished rapidly in the field andimmediate testing of various camweight configurations can be conducted.In this manner, optimum tuning of the clutch can be accurately andinexpensively achieved, whereas prior tuning methods are so laborintensive and time consuming that the precise engine parameters are noteasily verified and altered, thus resulting in tuning outside of theoptimum range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side sectional view showing a variable speed belt driveemploying an adjustable camweight constructed according to theprinciples of the present invention;

FIG. 2 is an exploded view of the driving clutch assembly depicted inFIG. 1;

FIG. 3 is a perspective view of a first embodiment of a camweightconstructed according to the principles of the present invention;

FIG. 4 is a is rear elevation of the camweight depicted in FIG. 3;

FIG. 5 is a side elevation of the camweight depicted in FIG. 3;

FIG. 6 is a perspective view of a second embodiment of a weight armconstructed in accordance with the principles of the present invention;

FIG. 7 is a rear elevation of the weight arm depicted in FIG. 6;

FIG. 8 is a side elevation of the weight arm depicted in FIG. 6;

FIG. 9 is an expanded view of a second embodiment of an adjustablecamweight system constructed in accordance with the principles of thepresent invention;

FIG. 10 is a rear elevation of the camweight system depicted in FIG. 9;and

FIG. 11 is a side elevation of the camweight system depicted in FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A variable speed belt drive 1 is shown generally in FIG. 1. The beltdrive 1 includes a driving clutch assembly 2, a driven clutch assembly 3and a flexible belt 4 extending between the two clutch assemblies 2 and3. An engine 5 drives a crankshaft 6 which is rigidly attached to afixed sheave 7. The fixed sheave 7 is formed to include a slanted outerwall 10. The fixed sheave 7 is rigidly connected to a shaft 8 upon whichis mounted a movable sheave 9. The movable sheave 9 is also formed toinclude a slanted outer wall 11. Thus, as the outer wall 11 of movablesheave 9 travels in the direction of arrow 12, the region 13 of belt 4that resides between the slanted walls 10 and 11 is forced outwardly inthe direction of arrow 14. This movement of region 13 has the effect ofincreasing the effective radius and hence the circumference of the pathalong which the belt 4 must travel.

Travel of the movable sheave 9 is attributable to several factors.First, a pressure spring 15 resides between inner wall 16 of the movablesheave housing 17 and the spider tower 18. The spider tower 18 has alower end 19 to which is attached a pivotable torque transfer button 20.The torque transfer button 20 is free to slide along a bearing surface21 in response to the resultant force exerted by components includingthe pressure spring 15.

The movable sheave housing 17 is formed so as to include an innersurface 24 which is the surface underlying the outer wall 11. Pivotablymounted to the inner surface 24 is a camweight 25. The upper end 22 ofthe spider tower 18 is formed to include a pivotable spindle 23 whichabuts the outer surface 26 of the camweight 25. As the crankshaft 6rotates at a given rate, the movable sheave 9 rotates at the same rate.As the rate of rotation increases, the free end 27 is flung outwardly,generally in the direction of arrow 28. This outward movement of thecamweight 25 causes the camweight surface 26 to exert a force againstspindle 23, thereby urging the spider tower to move in the direction ofarrow 28 and creating a reaction force against inner wall 24 that actsin the direction of arrow 12. When the urging force of the camweight isgreater than the opposite force exerted by pressure spring 15, theseparation between sheave outer walls 10 and 11 decreases, therebyincreasing the path length traveled by belt 4. The belt 4 thus transfersa variable rate of rotation to the drive clutch assembly 3, and therebytransfers the rotation of crankshaft 6 to the vehicle drive or outputshaft 29.

Referring now to FIG. 2, additional details concerning the constructionof driving clutch assembly 2 will be apparent. Most significantly, themovable sheave 9 is formed to include three substantially identicalpivotable camweights 25, 30 and 31, which are mounted at 120° spacingswithin the circumference 32 of movable sheave 9. The spider tower 18includes three distinct lobes 33, 34 and 35, each of which is formed toinclude a discrete torque transfer button 20, 36 and 37, respectively.Each of the lobes is acted on by a separate camweight. Therefore,camweight 25 acts against lobe 35, camweight 31 acts on lobe 33 andcamweight 30 acts against lobe 34. In alternate designs, the spidertower may include, for example, four distinct lobes, thereby requiring90° spacings of the camweights. One can appreciate that each of thecamweights 25, 30 and 31 should have virtually identical characteristicsin order to achieve proper dynamic balance during engine operation. Inorder to remove the actual camweight 25, for example, the bolt 38 andnut 39 must be removed. A similar operation must occur for the remainingtwo camweights 30 and 31, and then each weight must be replaced and thenuts and bolts refastened.

As best seen in FIGS. 3-5, the first preferred embodiment of a camweight40 according to the present invention includes a head 41 through whichis formed a mounting bore 42. The outer surface 43 is generally curvedand adapted for engagement with a spindle mounted on the spider tower.The outer end 44 is typically planar so that each cross sectional volumeof the camweight 40 may be readily calculated. The inner surface 45 ofthe weight 40 may be planar or may include a hump 46 so as to obtain thedesired moment of inertia characteristics.

Two methods of weight and moment of inertia adjustment are provided forcamweight 40. First, a series of perforations 47, 48, 49 and 50 areformed near the bottom surface 44 of weight 40. When it is desired toremove mass from the arm 40, a cutting tool (not shown) can bepositioned to engage the desired perforation and snip or cut off thedesired segment. Typically, the tool would initially engage perforation47, thereby removing segment 51 of the arm 40. The vehicle would then beoperated to evaluate the effect on transmission performance, andadditional segments could be removed as necessary.

A second method of weight adjustment is available when mass must beadded to the arm 40. A series of bores 52, 53, 54 and 55 are formed intothe arm 40 during its manufacture. When mass must be added to the arm 40in the field, a suitable molten material, such as lead or tin, is pouredinto a bore, such as bore 55, and allowed to cure, a process thattypically take less that ten seconds. The vehicle is then operated todetermine the effect on transmission operation, and additional bores maybe filled if required.

Referring now to FIGS. 6-11, a second embodiment of the presentinvention will now be discussed. A parent weight arm 56 is formed toinclude a head 57 having a bore 58. The bore 58 is adapted to receivethe bolt 38 as shown in FIG. 2. The arm 56 is preferably formed of asintered metal in order to achieve uniform density characteristics, butother materials may also be used if their mass distribution ispredictable, and the arm can then be formed, for example, by aconventional CNC device. Joining the head 57 is a relatively slendercantilevered arm 59 having a varying cross section. The outer surface 60is curved so as to be engageable with the torque button 37. Formedwithin the cantilevered arm 59 is a series of countersunk bores 61, 62and 63. The bores 61-63 may be threaded, or they may be smooth dependingon the type of fastening system used. A series of shims or washers 64,65 and 66 may be attached to the cantilevered arm 59. Note in FIG. 7that the arm 59 is not centered with respect to head 57. This creates arelatively narrow region adjacent to surface 67 of arm 59 that isdefined by the volume formed by the intersection of the perpendicularprojections of surface 67 and the surface 68 underlying head 57.Ideally, no part of the cantilevered arm 59 or any attachments theretoshould extend beyond the edge 69 of head 57. Similarly, the oppositesurface 70 of arm 59 is one boundary of a volume defined by theintersection of its perpendicular projection with the projection ofsurface 71 which underlies head 57. In an alternate embodiment, the arm59 can be centered with respect to head 57, thereby creating regionshaving substantially equal projected volumes on either side of the arm59.

In order to adjust the mass of the arm 59, a shim 64, for example, maybe mounted to the surface 67. A series of shims similar to shim 64 buthaving varying thicknesses may be tried to obtain optimum transmissionperformance, with the thickest shim not exceeding the width of surface68. If less mass is needed, or the moment of inertia needs adjustment, ashim or washer 66 having less surface area may be used. Shim 66 may bemounted so as to overlie either bores 61 and 62 or bores 62 and 63.Either of shims 64 or 66 may be affixed to the surface 67 by means ofscrew 72, 73 and 74.

If substantially more mass must be added to arm 59 in order to achievethe desired results, a thicker or wider shim or washer 65 may be mountedso as to abut surface 70. If both shim 65 and shim 64 are to be mountedto the arm 59, the screw 72, for example, may have a hollow, tapped boreinto which the corresponding screw 75 may be inserted.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. An adjustable camweightfor a driving clutch assembly, comprising:(a) a head, the head having afirst width and a second width, the first width being the greatestlateral dimension of the camweight; the second width being less than 50%of the first width of the head; (b) a cantilevered arm, the cantileveredarm comprising:(i) a first end that abuts and is integrally formed withthe head; (ii) a moment of inertia; (iii) a plurality of laterallyextending bores; and (c) at least one mass possessing element, the masspossessing element being formed with a plurality of laterally extendingbores that can be aligned with the laterally extending bores of the armso as to facilitate mounting of the mass possessing element on the arm,the mass possessing element being affixed to the cantilevered arm so asto increase the mass of the camweight.
 2. The adjustable camweight ofclaim 1, wherein the arm is formed to have a first side surface and asecond side surface, the arm being adapted to simultaneously support ashim on the first side surface and the second side surface.
 3. Theadjustable camweight of claim 2, wherein a combined structure formed bythe arm, a first shim affixed to the first side surface and a secondshim affixed to the second side surface have a combined lateral width,the combined lateral width being no greater than the first width of thehead.
 4. The adjustable camweight of claim 3, wherein at least one shimcan be mounted in at least two positions on the first side surface,thereby permitting the moment of inertia to be adjusted while the massof the camweight remains substantially constant.
 5. A method ofadjusting the mass and moment of the inertia of a camweight, comprisingthe steps of:(a) forming the camweight so as to have a relatively widerhead region and a relatively narrower arm region; (b) affixing at leastone mass possessing element to the arm region; (c) forming a pluralityof laterally extending bores through the arm region; and (d) formingeach mass possessing element with a plurality of laterally extendingbores, at least some of the laterally extending bores of each masspossessing element being aligned with the laterally extending bores ofthe arm region.
 6. The method of claim 5, further comprising the step ofmounting at least two shims on the arm region.
 7. The method of claim 6,further comprising the step of reorienting a shim on the arm region soas to affect the moment of inertia of the camweight while the massremains relatively constant.